Photocopiers, fax machines, and scanners have many user interfaces. One example is a control to vary the darkness of the copy. The control may be labeled "lighter" or "darker" or use simple light and dark symbols to communicate to the users what the output will look like. However, computer or printed images are becoming more complicated. They may have gray scale images, and subtle backgrounds as well as text. Consequently, simple controls do not adequately communicate to the user how the resulting output will appear. There is a need therefore, for a simple control which can better communicate to the user the appearance of the expected output.
Photocopiers, fax machines, and scanners have many similarities. A major similarity is that they begin their process with an original image, typically on paper, and convert it to some intermediate electrical form prior to its ultimate reproduction. The reproduction is local in the case of a photocopier and remote, usually using telephone lines, in the case of the fax machine. A scanner is a device that encodes an original image into some type of electrical signal which is then passed to another device for display such as on a computer, processing such as enhancement, storage, or reproduction. A scanner is a part of most modern photocopiers and fax machines. Therefore, in the following discussion, the word scanner can also be understood to mean that component of a photocopier or fax machine that converts an original image to some intermediate electrical form. Some scanners convert the light and dark areas of the original image into a continuously variable voltage or frequency signal and then use this voltage or frequency to control the reproduction of a copy. A continuously variable signal such as this is called an analog signal. Many scanners produce a digital output instead of an analog output. A digital output consists of a number of discrete steps. A common example is encoding a digital signal in eight binary bits which can represent a value ranging from 0 to 255. Although this might appear as very limiting compared to a continuously variable analog signal, there are major advantages. A digital signal is more suited than analog to computer storage, mathematical manipulation or regeneration. This discussion uses digital examples since digital signals are more common in scanners today. However, the present invention also is well suited for use in those applications which employ analog signals.
Many times there is a need to manipulate an intermediate electrical signal created by a scanner. This manipulation can lighten or darken the ultimate reproduction, or use more sophisticated mathematical methods to enhance various aspects of the reproduction. A common example familiar to photocopier users is the light/dark adjustment on the photocopier control panel. Typical types of manipulation are introduced with a concept called a tone map.
A tone map is a graphical representation of a transformation that takes a tonal input and converts it to an output. In the tone map of FIG. 1, the input axis 10 and the output axis 11 both range from 0 to 255. This is a common range since 0 to 255 can be represented in eight binary bits. FIG. 1 represents an identity tone map where an input of N (0&lt;=N=&lt;255) maps to an output of N, indicated by the diagonal line 12. Values of N between 0 and 255 generally represent shades of gray from black (=0) to white (=255). Given the tone map of FIG. 1, an image from a scanner which passes through the tone map to a printer, in theory produces the same shades as the scanned original, within the limitations of the scanning and printing process. The characteristic of maintaining the same shade from input to output is illustrated in FIG. 1. A shade of medium gray, having an input value of 128 indicated by vertical line 13 is transformed to an output value of 128 indicated by horizontal line 14. One skilled in the art will recognize that other digital ranges larger or smaller as well as analog ranges are also possible. While this discussion deals only with tone maps for gray scale from black to white, all the concepts are readily applied to color as well. In the case of color, there are three tone maps, for a Red, Green Blue system (RGB) or four tone maps for a Yellow, Magenta, Cyan, black (YMCK) system.
FIGS. 2 and 3 show modifications to the straight line transformation of FIG. 1. The line 12 in both FIGS. shows the original identity transformation of FIG. 1, while the heavier lines 20 and 30 represent new transformations based on brightness. FIG. 2 corresponds to a brightness transformation with the brightness function reduced so that the output is darker than the original input. The transformation depicted in FIG. 2 is useful for reducing the brightness of originals that are too light. For example, the tone map of FIG. 2 transforms an input of 128 indicated by line 23 to an output of 64 indicated by line 24. Thus, the tone map of FIG. 2 transforms gray (128) to a darker gray (64). FIG. 3 represents the opposite situation. The transformation of FIG. 3 has the effect of lightening an input image. This corresponds to increasing a brightness control. For example, the transformation of FIG. 3 transforms an input of 128 indicated by line 33 to an output of 192 indicated by line 34. Thus, the tone map of FIG. 3 transforms gray (128) to a lighter gray (192).
FIGS. 4 and 5 show modifications to the straight line transformation of FIG. 1. The light line in both FIGS. 12 shows the original identity transformation of FIG. 1, while the heavier lines 40 and 50 represent new transformations based on contrast. The transformation of FIG. 4 accepts an input range of 0 to 255, but only outputs a range of about 50 to 200. This, in effect, compresses the gray scale tones to output a low contrast image. In a low contrast transformation, several input gray scale values get mapped into a single output gray scale value resulting in a loss of fine shading detail. Conversely, FIG. 5 represents a high contrast transformation. The input range from about 96 to 160 produces an output range of about 60 to 212. This accomplishes more than a two to one expansion of the input range of 65 points to an output range of 153 points. Such an expansion produces a high contrast transformation that emphasizes the input range around the center at the expense of the ranges at the high and low ends of the input scale. A high contrast transformation like this is useful when there is a need to amplify the tonal differences in a certain input range of the image.
FIGS. 2, 3 and 5 also illustrate another type of transformation called clipping. Clipping occurs when the line representing the transformation becomes horizontal. FIGS. 6 and 7 demonstrate the two types of clipping called white clipping and black clipping respectively. The transformation of FIG. 6 accepts an input range of 0 to 255, but any input from 193 to 255 results in an output of 255. This represents a compression of the input values from 193 to 255 and an expansion of the tones from 0 to 192. white clip is useful for inputs where the lighter shades from 193 to 255 do not carry any useful information. For example, in other scanning situations such as photocopy or fax, white clip can clean up the background associated with the imperfections in the original. Conversely, FIG. 7 represents a black clip transformation. The transformation of FIG. 7 accepts an input range of 0 to 255, but any input from 0 to 63 results in an output of 0. This represents a compression of the input values from 0 to 63 and an expansion of the tones from 64 to 255. Black clip is useful for inputs where the darker shades from 0 to 63 do not carry any useful information. For example, in other scanning situations such as photocopy or fax, black clip can convert the darker tones ranging from 0 to 63 of an original to black, and expand the dynamic range of the lighter tones from 64 to 255.
FIGS. 8 and 9 show example transformations of low and high gamma respectively. In FIG. 8 lower input values from 0 to 64 all generate an output very close to 0. This has an effect of compressing the darker shades more to black. The values of 192 to 255 however, are mapped into values of 100 to 255. This has the effect of expanding and emphasizing the lighter shades. While the white and black clipping of FIGS. 6 and 7 or the contrast of FIG. 4 did not use all of the input range, the gamma function uses all of the input range, but maps it unequally to the output. FIG. 9 shows another example of this unequal emphasis. In FIG. 9 low input values near 0 are assigned output values from 0 to 63 thus making the darker shades lighter and emphasizing even small differences between black and the dark grays. The input values of 96 to 255 however, are mapped into output values of 192 to 255. This has the effect of compressing and de-emphasizing the differences among white and the lighter grays.
Brightness, contrast, white clip, black clip and gamma are interactive in that the adjustment of one value may affect the results of another value. Collectively, these parameters are herein referred to as transformation parameters.